As a polyethylene filament with high strength, there is known a filament which is produced from an ultra-high molecular weight polyethylene by a so-called gel-spinning method and which has such a high strength and such a high elastic modulus that any of conventional filaments has never possessed, as disclosed in , JP-B-60-47922, and this filament has already come into industrially wide use.
JP-B-64-8732 discloses a filament which is made from an ultra-high molecular weight polyethylene having a weight-average molecular weight of at least 600,000 as a starting material by so-called “gel spinning method” and which has a higher strength and a higher elastic modulus than any of conventional filaments.
A high strength polyethylene filament produced by melt spinning is disclosed in, for example, U.S. Pat. No. 4,228,118. According to this patent, the high strength polyethylene filament disclosed is obtained by extruding a polyethylene having a number-average molecular weight of at least 20,000 and a weight-average molecular weight of less than 125,000 through a spinneret which is maintained at the temperature between 220 and 335° C., then taking over the polymer at the rate of at least 30 m/min. followed by drawing it at least 20 times at the temperature between 115 and 132° C. Thus the filament has a tenacity of at least 10.6 cW/dTex.
Moreover, JP-A-08-504891 discloses a high strength polyethylene filament which is produced by melt spinning polyethylene with high density through a spinneret, cooling the filament coming out from the spinneret, and then drawing the obtained fiber at the temperature of 50-150 C.
Since a high strength polyethylene filament by gel spinning was invented, the filament has been used in all fields, and the physical properties required for the high strength polyethylene filament as a raw material became still higher in recent years. In order to deal with a wide range use, i.e. to satisfy the required performance which accompanies each use, it is required to fulfill simultaneously that in any monofilament fineness, a filament should excel in mechanical strength and an elastic modulus, the filament should be uniform, and also there should be no fusion between each monofilament, etc. For example, as far as applications such as battery separators are concerned, a high strength polyethylene filament with small single yarn fineness is desired. By contrast, for ropes or nets with which a fuzz, a rubbing and the like (a so-called wear resistance) pose a problem, the one where single yarn fineness is to some extent thicker conversely is desirable.
Although it is tried to produce a high strength polyethylene filament by the so-called melt spinning, a high strength polyethylene filament which satisfies all the above-mentioned performances has not yet been obtained. It is possible to obtain a high strength polyethylene filament by using gel spinning on the other hand. However, due to the fact that a high strength polyethylene filament with a low monofilament fineness obtained with gel spinning had many fusions and press-stickings between each monofilament, the fiber fused and stuck by pressure became thickness nonuniformity to be a defect so that such a problem as a deterioration of the physical properties of a nonwoven fabric arose when this filament was used for a nonwoven fabric particularly with a low weight (METSUKE). Moreover, when the apparent diameter of the filament became thick caused by the filament fused and stuck by pressure, there was a problem such that the retention of knot strength and loop strength falls.